The present invention relates to photoacoustic analysis of a sample using a photoacoustic sample vessel. As used herein, the sample is a material that may be a gas, liquid, solid, or combination thereof including supercritical fluid, slurry, and suspension.
Photoacoustic or optoacoustic methods are used as a tool in many applications to obtain chemical and physical information on the properties of materials in solution, solids, or gases. Absorption of irradiation, for example electromagnetic, by a material generates an electronically/vibrationally-excited state intermediate. Several channels of excited state xe2x80x9cdeactivationxe2x80x9d lead to an observable photo-induced acoustic signal including (1) heat released from the excited state as it relaxes back to the ground state, (2) volume changes in the material due to absorption of light, and/or (3) electrostriction of the media surrounding the material due to polar changes in the excited state.
All three phenomena of heat, volume, and electrostriction generate an acoustic wave that is typically detected with a piezoelectric transducer. A photoacoustic vessel couples the light into the material and couples the resulting acoustic wave to the transducer.
Referring to FIG. 1, a typical photoacoustic vessel 100 comprises an acoustic detector 110 and an acoustic coupler 102 (usually known as a cuvette) that provides a chamber for holding a sample 108. The acoustic coupler 102 has a first body 104 (vertical sides or walls) and a second body 106 (bottom). The first body 104 cooperates with the second body 106 (by being attached) to form the chamber. The acoustic detector 110 is mounted on the second body 106. Thus, upon introduction of electromagnetic energy 112, for example light, an acoustic signal 114 is generated within the sample 108 and propagates through the sample 108 into and through the acoustic coupler 102 to the acoustic detector 110.
A disadvantage of the cuvette type photoacoustic vessel is a requirement for a relatively large volume (milliliters) of sample 108 required to perform the measurement. Specifically, in the life sciences fields, new biological samples may be extremely expensive or difficult to obtain, synthesize, or purify. Thus, there is a need for photoacoustic vessels having smaller sample volumes for obtaining important data. Another disadvantage of this technique is the limitation of the sample 108 being at atmospheric pressure due to the chamber provided by the cuvette being open to the environment.
FIG. 2 illustrates an improved photoacoustic vessel 100xe2x80x2 described in U.S. patent application Ser. No. 09/105,781 now U.S. Pat. No. 6,236,455, filed Jun. 26, 1998, incorporated by reference herein to the extent not inconsistent with the disclosure herewith. The photoacoustic vessel 100xe2x80x2 comprises an acoustic detector 110 and an acoustic coupler 102xe2x80x2 (referred to as a prism cell) that provides an enclosed chamber for holding a sample 108. The acoustic coupler 102xe2x80x2 has a first body 104 and a second body 106 with a shim 200 or spacer therebetween defining the chamber containing the sample 108. The shim 200 contacts a perimeter of the first and second bodies 104,106 as shown thereby forming a perimeter wall(s). The first body 104 forms a first wall of the chamber and the second body 106 forms a second wall of the chamber. The second body 106 has an acoustic detector 110 mounted thereon. Thus, upon introduction of electromagnetic energy (e.g., light) 112 an acoustic signal 114 is generated within the sample 108 and propagates through the sample 108 into and through the second body 106 to the acoustic detector 110. Sample volumes of less than 1 ml and as low as 0.1 ml have been achieved with this device.
High-pressure time-resolved photoacoustic studies have been done and obtained results for the kinetics of photo-generated meta-stable intermediates ([a] E. F. Walsh, M. W. George, S. Goff, S. M. Nikiforov, V. K. Popov, X.-Z. Sun, and M. Poliakoff, Energetics of the Reactions of (n6-C6H6)Cr(CO)3 with n-Heptane, N2, and H2 Studied by High-Pressure Photoacoustic Calorimetry, J. Phys. Chem., 100 (1996) 19425-19429; [b] E. F. Walsh, V. K. Popov, M. W. George, and M. Poliakoff, Photoacoustic Calorimetry at High Pressure: A New Approach to Determination of Bond Strengths. Estimation of the Mxe2x80x94L Bond Dissociation Energy of M(CO)5L (M=Cr, Mo; L=H2, N2) in n-Heptane Solution, J. Phys. Chem., 99 (1995) 12016-20). Pressures up to 130 bar (2000 psi) were reported. In addition to the upper pressure limitation, these systems were not conducive to obtaining time-resolved data, limited to optically dense samples, and had no flow-through capability.
Thus, in spite of these advances, there continue to be applications requiring yet smaller sample volumes (e.g., nanoliters). In addition, because photoacoustic methods generally are highly sensitive but have low selectivity, there remains a need for obtaining dynamic photoacoustic data to enhance the selectivity of the opto- or photo-acoustic technique.
The present invention is an improved photoacoustic vessel and method of photoacoustic analysis. The photoacoustic sample vessel comprises an acoustic detector, an acoustic couplant, and an acoustic coupler having a chamber for holding the acoustic couplant and a sample. The acoustic couplant is selected from the group consisting of liquid, solid, and combinations thereof. Passing electromagnetic energy through the sample generates an acoustic signal within the sample, whereby the acoustic signal propagates through the sample to and through the acoustic couplant to the acoustic detector.
In another embodiment of the present invention, the photoacoustic sample vessel further comprises a sample container, wherein the sample is contained within the sample container and the acoustic signal propagates through the sample into and through the wall of the container into and through the acoustic couplant to the acoustic detector.
In another embodiment of the present invention, the sample is pressurized above atmospheric pressure.
It is an object of the present invention to provide a photoacoustic sample vessel that uses a small sample volume.
It is a further object of the present invention to provide a method of photoacoustic analysis of samples at elevated pressure.
It is a further object of the present invention to provide a method of dynamic photoacoustic analysis.
An advantage of the present invention is the ability to use substantially reduced sample volume needed for a photoacoustic measurement. Specifically, the present invention is capable of using a sample volume of less than 0.1 ml and as low as a few nanoliters. An additional advantage is the capability to perform measurements under elevated pressures as great as 60,000 psi. Further, it is possible to obtain time-resolved data (dynamic signal analysis for greater selectivity) with the present invention. Analysis of the shape of the acoustic waveform provides the selectivity while analysis of the amplitude provides the sensitivity.
The subject matter of the present invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. However, both the organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following description taken in connection with accompanying drawings wherein like reference characters refer to like elements.